Microprogrammed Control Unit

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Microprogrammed Control Unit
Control Memory Sequencing Microinstructions
Microprogram Example Design of Control
Unit Microinstruction Format
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Control Unit

• Initiate sequences of microoperations

• Control signal (that specify microoperations)in a
  bus-organized system

groups of bits that select the paths in
multiplexers, decoders, and arithmetic logic units

• Two major types of Control Unit

Hardwired Control in Chap. 5

• The control logic is implemented with gates,
  F/Fs, decoders, andother digital circuits
• Fast operation.
• -Wiring change(if the design has to be modified)

Microprogrammed Control in this Chapter

• The control information is stored in a control
  memory, and the control memory is programmed to
  initiate the required sequence of microoperations
• Any required change can be done by updating the
  microprogram incontrol memory.
• -Slow operation

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Control Word
The control variables at any given time can be
represented by a string of 1s and 0s.
Microprogrammed Control Unit
A control unit whose binary control variables are
stored in memory (control memory).
Microinstruction
The microinstruction specifies one or more
microoperations
Microprogram
A sequence of microinstruction
Dynamic microprogramming Control Memory RAM

• RAM can be used for writing (to change a writable
  control memory)
• Microprogram is loaded initially from an
  auxiliary memory such as a magnetic disk
• Control words in ROM are made permanent during
  the hardware production.

Static microprogramming Control Memory ROM
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Microprogrammed Control Organization
Control Memory
A memory is part of a control unit Computer
Memory (employs a microprogrammed control unit)

• Main Memory for storing user program (Machine
  instruction/data)
• Control Memory for storing microprogram
  (Microinstruction)

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Control Address Register
Specify the address of the microinstruction
Sequencer
Determine the address sequence that is read from
control memory Next address of the next
microinstruction can be specified several way
depending on the sequencer input.
Sequencing Capabilities Required in a Control
Storage

1. Incrementing of the control address register
2. Unconditional and conditional branches
3. A mapping process from the bits of the machine
   instruction to an address for control memory
4. A facility for subroutine call and return

Control data register
Hold the microinstruction read from control
memory Allows the execution of the
microoperations specified by the control word
simultaneously with the generation of the next
microinstruction
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Selection of address for control memory
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Conditional Branching
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Mapping of Instructions
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Mapping of Instructions to Microroutines
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Microprogram Example
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Machine Instruction Format
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Symbolic Microinstructions
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Symbolic Microprogram- Fetch routine
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Symbolic Microprogram- Indirect routine
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Symbolic Microprogram
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Binary Microprogram
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Design of control unit
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Microprogram Sequencer
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Microprogram Sequencer
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